This application claims priority to PCT/FR99/02088 filed Sep. 2, 1999 and to FR 98/11005 filed Sep. 3, 1998.
The present invention relates to a capillary surface injection squeegee for screen printing liquid products of a viscous liquid under optimum conditions through the openings of a screen printing stencil. This mechanism can be adapted to the object of FR 2754473, or any other existing device with an incorporated reservoir in which a pressure can be adjusted.
Conventionally, liquid products are deposited on a surface by spraying, by immersion, or by the use of a brush when there are no complex areas to be protected. Otherwise, that is to say if there are complex areas to be protected, then the screen printing process is used. Firstly, the screen printing process consists of a stencil, or screen printing mask, which is usually made of a piece of synthetic material or stainless steel. This screen printing stencil is produced in such a way that it has blocked areas and open areas corresponding to the patterns to be reproduced. Secondly, the screen printing screen is placed on the surface to be screen printed, and an inclined squeegee which is pressed simultaneously and moved in a direction parallel to the screen forces the liquid product to pass through the open areas of the screen. Squeegees are generally made either of a profile member of more or less hard rubber, or of stainless steel plates. In all cases, the transfer pressure transmitted to the product depends upon the angle which is formed between the plane of the screen and the active face of the squeegee. The smaller the scraping angle, the greater the transfer pressure. As for the masks, these can be made of a woven mesh of stainless steel, polyester, or silk, or even stencil plates. These screens can be either flat, or cylindrical, in the case of continuous screen printing. The screen printing process, as described hereinabove, is a very old process but it has a certain number of disadvantages, such as the rheological changes in the product which is to be transferred during the work, either by the evaporation of solvents contained in the product, or by interaction with the ambient environment. This rheological change is translated by an obligation to readjust working parameters or to replace the altered product by a good quality product.
Another disadvantage is the fact that the transfer pressure applied to the product is difficult to control. Indeed, it is not enough to control the scraping angle since the transfer pressure is interdependent on the scraping speed, on the quantity of product found to the front of the squeegee, and on the rheology of the product which is itself variable while work is being carried out, as explained hereinabove. Furthermore, since the maximum transfer force which can be applied is low, the resulting maximum scraping speed as well as the rate of product flow are also limited.
In order to partly eliminate this lack of control over the transfer pressure, it is possible to use a coating operation which consists in spreading the product uniformly over the surface of the screen, and in thus making it available for the scraping operation strictly speaking. Although this additional operation improves screen printing quality, it has the disadvantage of placing a large surface area of the product in contact with the ambient environment, thereby giving rise to rapid changes in rheology by evaporation of the solvents contained in the product.
In order to correct the problems related to the changes in rheology and poor control of the transfer pressure, a certain number of devices have been proposed. These devices all consist in confining the product to a chamber and in expelling it under pressure through a very narrow distribution slit in contact with the screen.
WO 96/20088 and U.S. Pat. No 4,622,239 describe devices of this type which are suitable for the transfer of pasty products, such as solder pastes. These devices are not used to transfer low to high viscosity liquid products because the sealing provided by the members which delimit the distribution slit is inefficient because it is very wide and therefore each time that the device is removed from the screen, e.g. to change series, the product contained in the device continues to flow out, resulting in significant losses of product.
FR 2754473 proposes another device which has better sealing but in which significant losses of product have also been noted as soon as an attempt is made to transfer liquid products, and in this case as well, the product contained between the sealing elements flows out from the device when it is removed from the screen. The invention described and claimed is also limited to use with pasty products, such as solder pastes.
U.S. Pat. Nos. 3,921,521, 4,023,486, GB 1,433,957 and DE 2250092 also relate to closed transfer devices, which, this time, are dedicated to liquid products, but in these cases too all of these devices have a distribution chamber which is delimited at the bottom by the sealing members which form a narrow slot closed by the printing screen, which means that the product contained inside that chamber flows out when the devices are removed from the screen.
In all the afore-mentioned patents, it is noted that product leaks particularly in the areas of the screen not supported by the substrate, e.g. at the sides when this latter is narrower than the length of the product distribution slit, or even at the start or end of the scraping operation when the screen printed substrate is replaced with a new substrate. All these instances of leakage, irrespective of their cause, are particularly harmful, because not only do they produce significant losses of product; they can also interfere with the quality of the deposits. Indeed, when the product which has been exposed to ambient air happens to be located on the surface of the screen, and is therefore unintentionally transferred onto the substrate during subsequent screen printing, this can cause deposits which lack precision or which are of poor quality. Moreover, excess product on the screen causes prolonged downtime for cleaning, and poses health and safety problems for the staff who come into contact with said products, particularly in the case of highly volatile and hazardous solvent-based inks like toluene.
Within a domain which differs significantly from that of the invention, namely the domain of photo-engraving and/or of flexographic printing, DE 43 30 681 proposes that to coat a product on an impression roller an applicator device is used which comprises a capillary element impregnated with coating product which is in direct and constant contact with the coating roller. The purpose of this capillary element is to provide homogeneous coating of the roller with which it is in contact. Although this device is interesting in its use of an intermediate capillary element it cannot be used within the domain of screen printing because this latter integrates the use of a screen printing screen which is responsible for transferring product properly onto the substrate for screen printing. Furthermore, the main inconvenience of the conventional processes and devices is the difficulty of preventing the product from flowing out when the applicator device is no longer in contact with the screen, if the product is particularly runny.
The present invention makes it possible for the various problems associated with prior art transfer devices to be remedied.
With respect to the device, according to the present invention, a member called a capillary surface injection member is placed between the closed reservoir containing the product to be transferred and between the screen printing screen in order to form deposits, by screen printing, of a liquid to liquid viscous product on a substrate through the openings in a screen which is resting on said substrate; the liquid product being available in a reservoir which can be pressurised and which is open on the side of the screen by a distribution surface whose dimension is adapted to the width of the substrate onto which the deposit is to be made; said reservoir which is integral with the capillary surface injection squeegee is mounted onto a screen printing machine, and, in conjunction with the pressurisation of the liquid, the reservoir assembly plus the capillary surface injection squeegee is subjected to a relative translatory movement with respect to the screen and is pressed onto this latter.
According to the main feature of the invention, the capillary surface injection squeegee is placed between said reservoir and said surface injection screen, incorporates a capillary element and has a greater or less flow resistance to prevent liquid from flowing out of the reservoir when it is not under pressure, and to allow it to flow out when it is under pressure. Furthermore, still in accordance with this main feature of the invention, said capillary surface injection squeegee is closed over the front portion and rear portion by means of sealing lips which are inclined at opposite angles and which make constant contact with said screen. Moreover, said squeegee is receptive to elastic deformation by the sealing lips thereof and contains an available volume of product which increases when pressure on said capillary surface injection squeegee is relaxed and which decreases when pressure is applied to said injection squeegee.
In conventional transfer members, in order to restrict to a maximum the natural flow of the liquid which is to be distributed, it is necessary to close up the distribution slit as much as possible by means of the lips; because of this, in order to provide a suitable flow it is necessary to increase the pressure on the product considerably, which is harmful locally particularly as far as the working life of the stencil is concerned and with respect to the sealing efficiency.
The invention consists in a means of employing the capillary effect in order to prevent the natural flow of product which is associated with a very wide distribution slit proposed by the squeegee of the invention whereof the very great surface area in contact with the screen printing stencil represents the injection location for reducing the pressure to be applied to the product whilst increasing flow rate.
According to the invention, this capillary surface injection squeegee is resistant to sufficiently fast flow (capillarity) to prevent the product which is to be transferred from flowing out of the device when this latter is removed from the screen printing screen, and to prevent the product from being pressurised. The resistance to flow (capillarity) of the capillary element can be adjusted depending upon the viscosity of the product to be transferred.
In accordance with the invention, the permeability of the capillary surface injection squeegee is consistent with the application requirements, that is to say that the flow rate of the product under pressure through the capillary surface injection squeegee can be adjusted in accordance with the desired rate of screen printing desired, the type of substrate, and the viscosity of the product to be transferred, it being understood that the flow through the capillary surface injection squeegee can also be adjusted by controlling the pressure applied to the product in the reservoir.
In accordance with the invention, the capillary surface injection squeegee has an available product-occupied volume which decreases if the pressure exerted to push the device into contact with the screen increases and which decreases if the force of the pressure is reduced. In this way, the residual liquid located at the distribution surface will be sucked back in by low pressure at the end of the scraping operation or when the device is removed from the stencil.
In accordance with the invention, the capillary surface injection squeegee has over the front and back two sealing lips which are inclined in opposite directions and which are responsible for scraping off excess product and thus confining it to the inside of the device. Additional sealing blocks placed on either side of the device make it possible product leakage at the ends of the capillary surface injection squeegee to be avoided.
In accordance with the invention, the capillary surface injection squeegee is flexible enough to be consistent with variations in height associated with irregularities in the substrate or screen, and thus allows the sealing lips which delimit the distribution surface to carry out their role in scraping out the excess.
It is important to emphasize the parity between the sealing lips and the capillary element which are the main elements of the squeegee according to the invention. This combination between the elasticity of said sealing lips and the volume occupied by the capillary element cannot figure in DE 43 30 681 since the lips described in said document are not elastic and do not change the volume of the capillary element by virtue of their elasticity.
Also, the use of an intermediate capillary element between the product to be screen printed under pressure and the screen printing screen is not a simple adaptation of a known device in a new domain of application but the fruit of studies and research for the purpose of satisfying a need which differs from that of homogeneous coating using an impression roller and which is rather to prevent screen printing product from being lost, or, at least, its initial rheological properties from being lost, by virtue of the integration of a capillary element which holds it in place when pressure on the product is interrupted.
In accordance with another particularly advantageous feature of the invention, said sealing lips which come into contact with said screen and the opening of which determines the injection surface area of the product are receptive to elastic deformation and are connected to said capillary element in such a way that its volume changes depending upon whether pressure is applied, or not, to said capillary squeegee.
Another object of the invention is constituted by the working process of the capillary surface injection squeegee which proposes to use the afore-mentioned features and consists in:
pressing said squeegee onto said screen in such a way that by virtue of the connection of the sealing lips to the capillary element and by virtue of their elasticity, the sealing lips reduce the volume of the capillary element, pressurise the product in such a way that this latter is able to pass through said capillary element, and subject said squeegee to a translatory movement relative to the screen in order to carry out the screen printing of a substrate which is placed underneath the screen, then
stopping movement, in eliminating pressure on the product in order that this latter may no longer pass through said capillary element, and interrupting the pressure of said squeegee on said mask in order that by virtue of the connection of the sealing lips to the capillary element and by virtue of their elasticity, the sealing lips increase the volume of the capillary element so that when screen printing is interrupted the low pressure created sucks in the product which is present directly in the vicinity of the injection surface. This ability to create a low pressure by releasing pressure on the squeegee provides a significant improvement with respect to keeping the rheological properties of the product to be screen printed, as well as in preventing the creation of a deposit and wastage of the product.
In accordance with another feature of the invention, the capillary surface injection squeegee constitutes an independent module which can be mounted on a transfer device comprising a reservoir which can be pressurised.
In accordance with another feature of the invention, all components of the capillary surface injection squeegee are chemically resistant to the products used as well as to cleaning solutions which may be used to clean the transfer device.
The description hereinafter gives two embodiments of the invention by way of non-limitative example which are illustrated in the drawings which will allow for a better understanding of it and reveal other advantages and features of the invention.